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Leaching Cone 6 Glaze Case Study

Section: Glazes, Subsection: Food Safety

Description

An example of how we can use INSIGHT software to determine of a glaze is likely to leach

Article

Following is an example of how we might look at a glaze and determine if it is food safe. Here is a letter we received:

"I have a receipt for a matte turquoise glaze which I am using on your M350 clay and I was wondering if it was food safe. The following is the recipe.

Turquoise Glaze
Dolomite          12
Kaolin             5
Gerstley borate   12
Potash feldspar   12   
Nepheline syenite 23   
Silica           5.5   
Strontium carb    15   
Ziropax           10   
Copper carb        5

Everyone I ask about the strontium carb. gives me a different answer about it's safety. I would like to be sure it is food safe before I use the glaze on functional pottery. I would appreciate an answer from you. Thanks."

It appears that strontium carbonate could be the least of this glazes problems. Here is the chemistry of the glaze as calculated by INSIGHT:

 CaO       0.32*
 MgO       0.21*
 K2O       0.09*
 Na2O      0.12*
 SrO       0.27*
 Al2O3     0.26 
 B2O3      0.11 
 SiO2      1.53 
 ZrO2      0.14

Other articles on this site talk about balance in the chemistry of glazes and SiO2 and Al2O3 content are of primary concern in this regard. This glaze is extremely low in SiO2 (it is 1.5)(recommended minimum is 2.5), that leads to unstable glasses that leach. You could simply increase the silica in this glaze but it is so low that it needs to be increased to 30 parts in the glaze recipe to supply the minimum amount of the oxide SiO2! That is certainly going to affect the appearance of the glaze.

Copper is a destabilizing influence in many glazes. Often non-leaching glazes will begin to leach after copper is added. This glaze has lots of copper. Leach testing is obviously needed.

The recommend maximum for SrO is around 0.2, this has 0.27, that is likely too high (INSIGHT has built-in limit formulas that assist in determining minimum and maximum amount for oxides).

Perhaps you would agree that the writer should go back to the people who said this glaze is safe and ask them what they were thinking. We cannot guarantee that it is not, but certainly a simple leaching test is advisable. I am guessing overnight in vinegar will leach out the color at least.

Out Bound Links
The future of ceramic recipe, material and physical testing record keeping is here.
Watch the video or sign-up at http://insight-live.com.

Maintain your recipe database on-line

  • Login to a private account or work with others in a group account (e.g. university).
  • Nothing to install (access it using your web browser). It is always the latest version.
  • Easy to import your existing data.
  • As many side-by-side recipes as you want.
  • Many ways to search and classify glaze and body recipes.
  • Glaze and body recipes are robust, with units-of-measure, unlimited pictures with individual titles and descriptions.
  • Add variations to a recipe; each with its own pictures, descriptions and name/code-number extensions.
  • Recipes can link to typecodes, projects and firing schedules (all managed in their own areas).
  • Standard reports and mix ticket reports with last-minute-totalling; variations report as if they are a complete recipe.
  • Video tutorials, help system, contact form on every page, dedicated messaging and support ticket systems.
  • It is an industrial-strength database system (unlimited capacity, fast, reliable, scalable).

Imports many file formats

  • Glaze recipe formats supported: HyperGlaze, GlazeGhem, GlazeMaster, Matrix, INSIGHT XML recipes (single and multiple), INSIGHT SQLite DB files.
  • Assign a batch number to imports, and later search by batch.
  • Assign multiple typecodes to imported glaze and body batches (to classify) and search on these later.
  • Prepend character sequences to glaze recipe names during import.
  • Import the pictures and pair them to their corresponding records automatically.
  • One click to automatically export the database to an SQLite DB database file and download it (for use with desktop INSIGHT or just as a backup).
  • Export and import individual glaze recipes as text or XML.

Perfect for Education

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